postgresql: fix encoding of pgctl path

This should be a regular string and not bytes. Otherwise in case of error
(pg_ctl not found), this happens:
  ERROR [pifpaf] sequence item 0: expected str instance, bytes found

Closes #118

pifpaf/drivers/postgresql.py

@@ -52,7 +52,7 @@ def _setUp(self):
         self.putenv("PGDATA", self.tempdir, True)
         self.putenv("PGDATABASE", "postgres", True)
         _, pgbindir = self._exec(["pg_config", "--bindir"], stdout=True)
-        pgctl = os.path.join(pgbindir.strip(), b"pg_ctl")
+        pgctl = os.path.join(pgbindir.strip().decode(), "pg_ctl")
         self._exec([pgctl, "-o", "'-A trust'", "initdb"])
         if not self.sync:
             cfgfile = os.path.join(self.tempdir, 'postgresql.conf')